The instant invention relates to hydropholic microporous membranes, of the type wherein a hydrophilic polyolefinic microporous material is impregnated with a surfactant in order to impart hydrophilic properties, such as water permeability, to the material. The present invention particularly relates to hydrophilic microporous materials of the aforementioned type which are rapidly wettable, particularly by aqueous solutions, and in addition may be multiply re-wet, i.e., re-used, without significant surfactant wash-out and an attendant loss of hydrophilic properties of the material upon re-wetting.
Polyolefinic microporous materials, such as for example polyolefinic microporous films and hollow fibers, are well known to those skilled in the art. See, for example, U.S. Pat. Nos. 4,020,230; 4,055,696; 4,290,987; 3,839,516; 3,801,404, 3,679,538; 3,558,764; and 3,426,754. Due to the advantageous properties of these materials, such as chemical inertness and stability, physiological compatibility and safety, as well as the unique porous structure thereof, polyolefinic microporous films and hollow fibers have found utility in a wide variety of applications such as gas-breathing water barriers, gas-liquid transfer mediums, and membranes for use in blood oxygenation and in various separation processes.
One disadvantage of the polyolefinic microporous membranes which in the past has limited the number of applications in which these materials may be utilized has been their hydrophobic nature. Due to the hydrophobic nature of these materials, despite their other desirable physical characteristics, the polyolefinic microporous membranes have not been readily usable in those applications which require an aqueous permeable and/or wettable membrane, such as for example battery separators and membranes for use in blood plasmaphersis.
One approach to this problem has been to treat the normally hydrophobic polyolefinic microporous material with various surfactants in order to impart hydrophilic characteristics such as water permeability and wettability thereto. U.S. Pat. No. 3,853,601, for example, discloses polyolefinic microporous films such as polypropylene microporous films, which are rendered hydrophilic by treatment with a silicon glycol copolymeric surfactant. In other embodiments, the microporous film may be impregnated with a combination of a silicon glycol copolymeric surfactant and a cationic imidazoline tertiary amine. The disclosed surfactants are applied to the exemplified polyolefinic microporous films by contactng the film with a dilute solution of from about 1 to 10% by weight of the surfactant and/or surfactants in an organic solvent such as acetone, methanol, ethanol or isopropyl in order to produce an "add-on" of the surfactant to the microporous film of from about 2 to about 20 percent by weight, based on the weight of the uncoated microporous film. Hydrophilic microporous polyolefinic films produced by this method are described as being rapidly wettable and useful as battery separators.
Similarly, U.S. Pat. No. 4,290,987 teaches that polyolefinlic microporous hollow fibers may be rendered hydrophilic by treatment with surfactants such as polyvinylpyrrolidone (PVP), various high molecular weight condensation products of ethylene oxide with a hydrophobic base formed by the condensation of propylene oxide with propylene glycol (e.g., the Pluronics.RTM.), and various polyoxyalkylene derivatives of hexitol anhydride partial long chain fatty acid esters (e.g., the Tweens.RTM.).
In addition to the foregoing surfactants, a variety of other surfactants have also been examined for use in rendering polyolefinic microporous materials hydrophilic. U.S. Pat. No. 4,298,666, for example, describes the use of phosphate esters such as ethoxylated 2-ethylhexyl phosphate in the treatment of polyolefinic microporous films.
A wide variety of various other surfactants which have heretofore been employed in order to improve the hydrophilic characteristics of polyolefinic films are further described in Canadian Pat. No. 981,991, such a hexachlorophene modified soaps, polypropoxylated quaternary ammonium chlorides, organic phosphate esters, imidazolines, fatty acids and their alkali metal salts, and various fatty and alkali amines.
Wettable polypropylene non-woven mats, suitable for use as battery separators, have also heretofore been developed by the prior art. U.S. Pat. No. 3,947,537, for example, describes non-woven polypropylene mats having a pore size of less than 25 microns and a porosity in excess of 50% which are rendered wettable by impregnation with from about 0.3 to 1.0 percent by weight of an anionic surfactant such as sodium dioctylsulfosuccinate or nonionic polyoxyethylene oxy compounds such as nonylphenoxy poly(ethyleneoxy) ethanol (i.e., Igepal.RTM. CO-730) or alkyarylpolyether alcohol compounds.
Similar battery separators of the porous non-woven mat type are also disclosed in U.S. Pat. Nos. 3,870,567; 3,918,995; and 3,933,525. In each of these patents, the non-woven mats are prepared from polypropylene fibers containing a wetting agent system that is insoluble in battery electrolyte and tends to bloom (i.e., rise to the surface of the polypropylene fibers) under conditions of use. In the preferred embodiment, the wetting agent system comprises a first nonionic wetting agent having an HLB (Hydrophilic Lypophilic Balance) number of less than 5, and a second wetting agent having an HLB of greater than 5 selected from nonionic and anionic surfactants. Specifically exemplified surfactants include, inter alia, the ethoxylated adducts of polypropylene oxide with propylene glycol (Pluronic.RTM. L-121), nonylphenol ethylene oxide adducts such as Tergitol.RTM. NP-14, TP-9, NP-33 and NP-44; dodecylphenol ethylene oxide adducts such as Tergitol.RTM. 12P-5 and 12-P-6, and alkylarylpolyethers such as Triton.RTM. X-100, as well as a variety of other surfactants based on ethylene oxide.
While the above-described hydrophilic polyolefinic materials are wettable, in varying degrees, with aqueous liquids and are thus suitable for some purposes, the surfactant impregnated polyolefinic materials of the prior art have in general exhibited less than desirable retention of their hydrophilic properties upon repeated use. It has been observed with these systems that many of the surfactants employed therein have a relatively low adhesion to the polyolefinic material, and upon repeated use tend to be washed-out from the polyolefinic material with an attendant loss in the hydrophilic and wettability properties of the polyolefinic material. In battery separator applications, for example, the long term bathing and agitation of the polyolefinic separators by the battery electrolyte produces surfactant wash-out, with a resulting increase in internal electrical resistance as the life of the battery increases. Similarly, in applications such as plasmaphersis (as well as in other filtration processes) passage of the blood plasma (or other liquid to be filtered) through the polyolefinic microporous membrane tends to wash-out the surfactant, with the result that the efficiency of the microporous polyolefinic membrane decreases percipitously over time.
Recently, hydrophilic microporous polyolefinic films have also been developed wherein the water wettability and water permeability properties of the hydrophobic polyolefinic material are improved by grafting the polyolefinic material with a controlled amount of a hydrophilic monomer such as acrylic acid in the presence of a controlled dosage of ionizing radiation. See, e.g., U.S. Pat. No. 4,346,142. Hydrophilic microporous polyolefinic films prepared by the technique disclosed in this patent, due to the chemical attachment of the hydrophilic monomer to the polyolefinic material, do not exhibit transient water permeability properties and thus comprise highly desirable hydrophilic membrane materials. Due to their convenience, it would nonetheless be desirable to provide hydrophilic microporous polyolefinic materials of the surfactant impregnated type having improved surfactant retention and re-wettability properties.